Underground storage tanks, which are typically made of fiberglass reinforced plastic, are well known in the art. Such tanks are commonly used to store gasolines and other fuels, as well as corrosive liquids. Typically, these tanks have a plurality of spaced-apart ribs around the circumference of the generally cylindrical tank. Recent innovations in such underground tanks include the development of a double wall tank. Double wall tanks provide a double barrier to prevent leakage of the liquids from the tank. Also, the annular space between the inner and outer walls can be filled with a leak detecting fluid which can be monitored to detect leaks in the tank's inner or outer wall. The annular space between the inner and outer tank walls can be connected to the hollow ribs in order to form a leak detecting space between the inner and outer walls.
It is known to use a plastic mesh material as a spacer to separate the outer tank wall from the inner tank wall, and to help define the annular space. Such mesh is typically polyethylene mesh. In tanks having a dry annular space, i.e., not filled with a leak detecting fluid, the bottom of the tank is usually adapted with a fluid detecting means. In such a tank, a leak in either the inner or outer tank walls introduces a fluid into the annular space, and this fluid flows downwardly within the annular space, eventually reaching the fluid detecting means at the bottom of the tank.
In tanks designed for the presence of a leak-detecting liquid permanently within the annular space, a leak in the inner or outer tank walls causes the level of the leak detecting fluid in the reservoir to either rise or fall, thereby activating an alarm signal.
An important consideration in manufacturing double wall underground storage tanks is the cost of manufacture. Also, it is imperative to have good fluid communication throughout the annular space so that a leak in any one part of the tank is communicated to the leak-detecting sensors. Therefore, it is important for good fluid flow paths to be established within the annular space, particularly in the vertical direction.
The plastic mesh material typically used as a separator is a woven or non-woven criss-cross mesh. This mesh, although performing quite well as a separator, requires a significant amount of labor to apply. It would be desirable for the annular space to be provided with a separator material which is a low-cost material, easy to apply, and enabling the relatively rapid flow of fluids within the annular space.